Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal thermography a visible image pattern is formed by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. Such recording materials become photothermographic upon incorporating a photosensitive agent which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
Examples of photothermographic materials are the so called "Dry Silver" photothermographic materials of the 3M Company, which are reviewed by D. A. Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43, published by Marcel Dekker in 1991.
EP-A 889 355 discloses a thermographic recording material comprising a substantially colourless support and a thermosensitive element containing a substantially light-insensitive organic silver salt, an organic reducing agent for the substantially light-insensitive organic silver salt in thermal working relationship therewith and a binder, characterized in that a blue pigment or dye having an absorption maximum in the wavelength range from 550 to 700 nm is present in the thermosensitive element and/or any other layer on either side of the support which provides a background for viewing in transmission images produced with the thermographic recording material.
U.S. Pat. No. 5,783,380 discloses a thermally processable imaging element comprising: (1) a support; (2) a thermographic or photothermographic imaging layer on one side of the support; (3) a protective layer overlying the image-forming layer; said protective layer comprising: (A) a film forming binder; (B) a dye dispersed throughout said protective layer in an amount sufficient to impart a pre-selected color thereto; and (C) matte particles the color of the protective layer. No disclosure is made in U.S. Pat. No. 5,783,380 regarding colorant in the support.
EP-A 919 864 discloses a photothermographic element comprising at least one photosensitive layer on a support, wherein the support contains a dye of structure I: ##STR1##
wherein M is a multi-valent metal atom; each of R.sub.1, R.sub.4, R.sub.5, R.sub.8, R.sub.9, R.sub.12 R.sub.10, and R.sub.16 independently represent a hydrogen atom, or a substituted or unsubstituted, branched or unbranched alkyl group; each of R.sub.2, R.sub.3, R.sub.6, R.sub.7, R.sub.10, R.sub.11, and R.sub.14 and R.sub.15 independently represent a hydrogen atom, a halogen atom, a substituted or unsubstituted, branched or unbranched alkyl group, a substituted or unsubstituted aryl group, a substituted or unsubstituted alkoxy group or a substituted or unsubstituted aryloxygroup; or one cr more of the adjacent pairs R.sub.1 and R.sub.2, R.sub.2 and R.sub.3, R.sub.3 and R.sub.4, R.sub.5 and R.sub.6, R.sub.6 and R.sub.7, R.sub.7 and R.sub.8, R.sub.8 and R.sub.9, R.sub.9 and R.sub.10, R.sub.10 and R.sub.11, R.sub.11 and R.sub.12, R.sub.13 and R.sub.14, R.sub.14 and R.sub.15 and R.sub.15 and R.sub.16 taken together may represent the atoms necessary to form a substituted or unsubstituted aromatic or heteroaromatic ring; and wherein the support has a peak absorption between about 660 nm and about 800 nm.
Imaging materials for medical applications are in general produced using a support with a particular blue pigment e.g. MACROLEX.TM. BLUE 3R from BAYER. The colour of such supports can be defined in terms of L*, a* and b* CIELAB-values which are determined by spectrophotometric measurements according to ASTM Norm E179-90 in a R(45/0) geometry with evaluation according to ASTM Norm E308-90. Representative supports used for medical imaging materials have CIELAB-a* values and -b* values given in the table below.
a* b* Dvis MEDICAL IMAGING MATERIAL SUPPORT 1 -7 -13.82 0.172 MEDICAL IMAGING MATERIAL SUPPORT 2 -7.22 -13.02 0.174 MEDICAL IMAGING MATERIAL SUPPORT 3 -6.86 -14.46 0.181 MEDICAL IMAGING MATERIAL SUPPORT 4 -7.92 -16.62 0.195
However, the background colour and the colour of an image is a combination of the colour of the support and the colour of the image background and the image of the particular material upon printing. In the case of conventional silver halide images the CIELAB-a* and -b* values of the image are virtually independent of image density, whereas this is not the case for thermographic recording materials based on organic silver salts and reducing agents.
The resolution of medical images, such as X-ray images, is of supreme importance in ensuring reliable diagnosis. It is desirable to improve further the diagnostic reliability of images produced with thermographic recording materials based on organic silver salts and reducing agents using a support with a particular blue pigment, whether produced by thermography or photothermography, thereby ensuring more reliable diagnosis.